Safety switching arrangement for electrically heated devices



De 5, 1944. F. E. FlNLAYsoN 2,354,433

SAFETY SWITCHING ARRANGEMENT FOR ELECTRICALLY HEATED DEVICES Filed July29, 1941 Fr-anh E. Fnlayson,

Wwf. wlw b9 His Attcrney.

Furthermore, in such safety switch which Patented Dec. 5, 1944 SAFETYSWITCHIN G ARRANGEMENT FOR ELECTRICALLY HEATED DEVICES Frank E.Finlayson, Ontario, Calif., assignor to General Electric Company, acorporation of New York Application July 29, 1941, Serial No. 404,451

8 Claims.

This invention relates to electric heating devices such as electricilatirons and more particularly to improved means for automaticallydisconnecting such devices from the power supply incase the operatorneglects to do so.

In the use of electric heating devices, such as electric fiatirons,damage resulting from iire or scorching may occur if the iron is leftunattended in the horizontal or ironing position while connected to thepower supply. In order to preclude the possibility of such damage it isnot uncommon to provide an automatic safety switching arrangement inconnection with the handle of the iron whereby the heating circuit isclosed when the handle is grasped and automatically opened when it isreleased. While such arrangements are effective to prevent damagereferred to above, they have certain operating disadvantages. Forexample, during an ironing operation an operator frequently lays theiron aside temporarily while arranging the articles to be i-roned. As aresult the handle is grasped and the safety switch operated at frequentintervals and the resulting sparking at the contacts causes undesirableand annoying radio interference. arrangements, since the safety switchshuts oil the heating current each time the iron is temporarily laidaside, the iron has a tendency to cool down to a point where theYironing operation may be rendered unsatisfactory. Also frequentoperationof the safety switch causes the contacts to deteriorate rapidlybecause of sparking.

An object of my invention is to provide a new and improved controlsystem for an electric heating device.

It is a further object ofrmyminvention to provide a safety switchingsystem operating in conjunction with a handle of an electric heatingdevice, such as an electric atiron, which operates to break the heatingcircuit at relatively infrequent intervals whereby radio interferenceresulting from contact sparking is greatly reduced.

Another object of my invention is to provide a is arranged to cooperatewith a heat control thermostat of an electrically heated iron inV such amanner that the contacts of the safety switchrarely, if `ayer, arerequired to break the heating circuit whereby` inexpensive' switchcontacts may be utilized and the life of the con'- tacts is greatlyextended.

Further objects and advantages of my invention will become apparent asthe following description proceeds and the features of noveltywhichcharacterize my invention will be pointed out with particularity in theclaims annexed to and forming a part of this specification.

Briefly, according to the illustrated embodiment of my invention, asafety switch, arranged heating circuit. Shutting olf of the heatingcur-y rent by the main thermostat permits the safety switch contacts toopen and the iron cannot be reenergized, even though the thermostatconta-cts close, until the contacts of the safety switch are againclosed by action of the operator. With this arrangement the heatingcircuit of the iron is broken only by the normal operation of thethermostat contacts which occurs at relatively infrequent intervalswhereby radio interference due to contact sparking is greatly reduced.

My invention will be better understood by reference to the followingdescription taken in connection with the accompanying drawing in whichFig. 1 is an elevation view, partly in section, of an electric latironembodying improved control apparatus constructed in accordance with myinvention; Figs. 2 and 3 are partial sectional views showing my improvedcontrol apparatus in difterent operating positions; Fig. 4 is across-section view of the handle of the atiron shown in Fig. 1 along theline 4-4 looking in the direction of rthe arrows; and Fig. 5 is adiagrammatic showing of the circuit connections of the atlron.

Referring to the drawing, I have shown my invention applied in one formto an electrically heated flatiron having a sole plate I0 that is formedof any suitable material such as cast iron or aluminum. Cast within thesole plate I0 is a sheathed heating element II preferably of the typedescribed in United States patent to C. C. Abbott, 1,367,341, datedFebruary 1, 1921. As shown this heat'ng element comprises a helicalresistance heating element I2 mounted in a tubular metallic sheath I3and supported in spaced relation with reference to the sheath by acompacted layer of insulating material I4 whichpreferably will be ofHeating elements of to the desired shape usually formed roughly in theshape of a horseshoe having its terminals I5 at the heel of the iron andits apex at the toe of the iron.

In order to maintain the temperature of the iron' Within predeterminedlimits, I provide a heat control thermostat or thermostatic switch I6having contacts connected in series circuit relation with the heatingelement I2 and mov;- able between open and closed positions inaccordance with the temperature of the iron in a Well-known manner.While any type of sultable thermostatic switch may be used, I haveillustrated one of the type disclosed and claimed in United StatesPatent 2,152,850 granted May 15, 1939, on the application of J. R.Campbell and assigned to the assignee of the present invention. Briefly,the thermostatic switch I6 described in the above-mentioned Campbellpatent comprises a bimetallic element I1 and two similar Z-shapedsupporting members I8 the ends of which are secured together andIastened in good thermal relation to the sole plate I0 by means of afastening screw 20. Extending outwardly from the upper end of thesupporting member I8 are a pair of switch arms 2I and 22 carryingcooperating contacts 23. The

and I9 upper switch arm 2I is resilient in construction and is arrangedto bias the contacts 23 to the closed circuit position. The switchingmember 2I also has a portion 24 extending beyond the contacts 2-3 whichis adapted to be engaged by one end of an L-shaped member 25 extendingoutwardly and upwardly from the free end of the bimetallic member I1.The portion of the bimetallic member I1 having the higher coelcient ofexpansion is on the lower side so that when the bimetallic memberbecomes heated to a predetermined temperature its free end movesupwardly whereby the end oi' the L-shaped member 25 engages the outerend 24 of the spring switch arm 2l and opens the contacts 23. Thetemperature adjustment of the thermostatic switch is effected by meansof an adjusting screw 26 mounted in the supporting arm I9. This screwcarries at its lower end a button 21 which bears against the resilientsupporting member I8. When the screw is adjusted the position of thesupport and consequently the position of the switch arms 2| and 22 ischanged. By adjusting the position of the switch arms 2I and 22 relativeto the bimetallic element I1 the temperature setting of the thermostaticswitch is changed.

The iron is provided with a cover 30 secured to the sole plate bysuitable fastening means not shown. The cover 30 has-a portion 3|extending rearwardly of the sole plate and forming a heel stand forsupporting the iron inl an upstanding position. `The iron is providedwith a handle 32 which is secured to the upper portion of the cover 30by means of suitable fastening means, one of which isfshown at 33.

In order to provide means for automatically disconnecting the iron fromthe power supply when it is left unattended in the horizontal or ironingposition, I provide a. safety switch 35 connected in series circuitrelation with the heating element I2 and the thermostatic switch IB. Thesafety switch 35 is mounted on the upper side of the handle 32 andcomprises a pair of cooperating contacts 35, one of which is carried onthe end of a relatively rigid contact arm 31, while the other contact iscarried on the end of a resilient contact arm 38 constructed ofbimetallic material for a purpose which will be described in detailbelow. The contact arms 31 and 38 together with a leaf spring member 33,the purpose of which will presently appear, are mounted on a atprojection 40 extending upwardly from the top surface of the handle 32by means of a screw 4I. An insulating washer 42 is provided formaintaining the contact arms 31 and 38 in the proper spaced relation sothat the contacts 36 are normally in the disengaged position. Also, aninsulating washer 42a is provided for preventing a short circuit betweenthe contact arms 31 and 38 through the screw 4I.

For the purpose of enabling the safety switch 35 to be closed manuallywhen the handle 32 is grasped by the operator and for maintaining theswitch closed when the iron is supported on the heel rest in anupstanding or non-ironing position, I provide a pivotally mounted handlelever 43. 'I'he handle lever 43 is pivotally mounted to the cover member30 by means of a pivot pin 44 which extends through the lever and issupported on an upstanding lug member 45 secured to the cover by thescrew 33. As best shown in Fig. 4, the handle lever has a U-shapedcross-section, the inner surfaces of the downwardly extending side wallsbeing arranged to slidably engage the vertical side walls of the handle32 and to form in conjunction with the handle an enclosed space 45 inwhich the switch 35 is located. The contour of the lever is so shapedrelative to the-handle 32 that a smooth gripping surface is providedwhich is adapted to be grasped by the operator during the ironingoperation. The spring 39 is arranged to engage the upper wall of thehandle lever 43 so that the lever is biased upwardly to the positionshown in Fig. 1. Extending downwardly from the handle lever is anabutment 46 which is adapted to engage the free end of the contact arm38. Thus when the handle is grasped by the operator during the ironingoperation the lever 43 is depressed against the bias of the spring 39and the abutment 48 bends the contact arm 38 downwardly so that thecontacts 136 of the safety switch are forced into engagement as shown inFig. 2. It will be noted that the end portion 41 of the handle lever 43engages an edge 48 of a cutaway portion in the handle 32 to limit thedownward movement of the handle. To limit the upward movement of thehandle a pin 49 extends through and is mounted in the verticallyextending side walls of the handle lever. The pin 49 projects through anelongated slot 50 in the handle 32 and engages the upper edge of theslot to limit the upward movement of the handle lever. The elongatedslot permits suiilcient travel of the pin 49 to accommodate the downwardmovement of the handle lever.

When the iron is supported in the upstanding or non-ironing position onthe heel stand 3I there is no danger from iire or scorching and it isdesirable to maintain the safety switch Z5 closed in this positionindependently of any action on the part of the operator so that the ironcan be left unattended in this position during initial heating andbetween ironing periods. To accomplish automatic closure of the safetyswitch 35 when the iron is in the upstanding position, the pivotallymounted handle lever 43 is provided with an abutment 5I which is adaptedto engage the supporting surface when the iron is supported in theupstanding position on the heel stand. In this position the abutment 5Iis so located relative to the pivot pin 44 that the weight of the ironcauses the handle lever to pivot to the closed circuit position of thesafety switch shown in Fig. 2, and hence the safety switch isautomatically maintained closed while the iron is in the verticalposition.

The circuit connections for the safety switch 35 of the thermostaticswitchA I6 and the heating element I2 are best shown in Figs. 1 and 5 ofthe drawing. 'I'he iron is energized from a suitable source of electricpower (not shown), the power supply conductors 52 and 53 being shown inFig. 1 as extending inwardly through one wall of the lower portion ofthe' handle lever 43. The power supply conductor 52 is connecteddirectly to the xed end of the bimetallic contact arm 38 while the otherpower supply conductor 53 is connected to one of theterminals I5 of theheating element I2. The contact arm'31 of the safety switch is connectedby a conductor 54 to one of the contacts 23 of the thermostatic switchI6 while the other of the contacts 23 is connected by a conductor 55(not shown in Fig. 1) to the other of the terminals I5 of the heatingelement I2. It will therefore be apparent that the safety switch 35,/the thermostatic switch I 6, and the heating element I2 are connectedin series circuit relation with the energizing circuit of the iron.

According to my invention, when the handle lever 43 is depressed and theswitch 35 closed by the operator during the ironing operation, meansresponsive to a flow of current through the heating element of the ironis provided for maintaining the switch 35 in the closed circuit`position independently of the handle lever. Thus, even though the handlelever is released, the switch 35 remains closed until the thermostaticswitch I6 opens its contacts to shut off the current to the heatingelement I2 whereupon the safety switch is permitted to return to itsnormal open circuit position. In the illustrated embodiment of myinvention, this is accomplished by the action of the bimetallic contactarm 38 of the safety switch. When the safety switch 35 is closed bydepressing the handle lever 43 the current flowing tothe heating elementI2 passes through the bimetallic contact arm 38 and heats it. Thebimetallic contact arm comprises an elongated resilient bimetallic barformed of two metals having different temperature coefficients ofexpansion such as Invar and steel rigidly secured together lengthwise asby welding. The bimetallic contact arm is arranged with the metal havingthe higher coeicient of expansion on top so that as the contact armbecomes heated by the passage of current therethrough the contact armbecomes stressed downwardly so as to maintain the contacts 36 of thesafety switch 35 in engagement independently of the action of themanually operated handle 43. The contacts of the safety switch willthereafter remain closed until the contacts 23 of the thermostaticswitch I6 open on the heat cycle of the thermostat to stop the flow ofheating current to the resistance element I2 and to the bimetalliccontact arm 38. VAt this time, if the handle lever 43 is in the releasedposition, the contact arm 38 will cool rapidly permitting the contactsto resume the normal open circuit position.

I have found that `by properly designing the bimetallic contact arm 38of the safety switch the contacts can be made to stay closed in aboutfifteen seconds maximum time after manual closing of the switch bydepressing the handle lever, and upon deenergizing of theheatingcirtacts 36 of cuit, as by opening of the contacts of thethermostatic switch when the handle lever 43 is in the releasedposition, the contacts of the safety switch will open in about twentyseconds maximum time. The factors influencing the design of thebimetallic contact arm include the resistance and dimensions of thebimetallic material and the amount of heating current drawn by theresistance element I2. The contact opening temperature of the safetyswitch should be well above the maximum ambient temperature under alloperating conditions to insure the opening thereof when the handle leveris released and the thermostatic switch contacts open to deenergize theheating circuit.

In operation, let it be assumed that theiron is cold and connected tothe power supply. As shown in Fig. 1 the contacts ofthe thermostatlcswitch will be closed but the contacts of the safety switch 35 will beopen due to the fact that the spring member 39 biases the handle lever43 to the released position. If now the handle lever 43 is depressedeitherby the action of the operator in grasping the handle or by placingthe iron in the upstanding non-ironing position in which it is supportedby the heel stand 3| and the abutment 5I, the safety switch 35 will beactuated to the closed position as shown in Fig. 2. The closure of thesafety switch completes the energizing circuit to the heating element I2whereupon itcommences to heat the sole plate I0. It will be observedthat the current owing through the heating element I2 also flows throughthe bimetallic contact arm 38 since they are connected in series circuitrelation. This causes the contact arm 38 to become heated locallywhereby it becomes stressed downwardly and acts to keep the contacts 36closed independently of the pressure exerted by the abutment 46 of thehandle lever. When the sole plate reaches the predetermined temperaturecorresponding to the control point for which the thermcstatic switch I6is set, the free end of the bimetallic bar I1 moves upwardly and engagesthe end portion 24 of the contact arm 2| and opens the contacts 23 todeenergize the heating element I2. Thereafter the contacts 23 of thethermostatic switch cyclicly open and close to maintain the temperatureof the sole plate within predetermined limits in a well-known manner.

Now let it be assumed that the operator releases the handle lever 43 andleaves the iron in the horizontal ironing position where it may dodamage if it is not disconnected from the power supply. Let it also beassumed that at the time the operator releases the handle lever thecontacts 23 of the thermostatic switch IB are in the closed or lowtemperature position, the thermostat being on the heating portion of theheating cycle. As soon as the operator releases the handle the spring 39will force the handle lever 43 upwardly to the released position. Theconthe safety switch 35, instead of opening at the time the handle leveris released and breaking the circuit to the heating element I2, remainclosed due to the fact that the bimetallic contact arm 38 has beenheated locally and is stressed downwardly by the action of the heatingcurrent flowing therethrough. Therefore, the abutment 46 moves out ofcontact with the free end of the contact arm 38 as shown in Fig. 3 ofthe drawing. When the temperature of the iron reaches the maximumtemperature of the thermostat heat cycle, the thermostat contacts 23move to the'open or high temperature posi- 4 tion to interrupt the fiowof current to the heating element l2 and also through the bimetalliccontact arm 38. After a short interval of time the bimetallic contactarm 38 cools and moves upwardly causing disengagement of the contacts 36of the safety switch 35, the safety switch returning to the positionshown in Fig. 1 of the drawing. The safety switch now being opened theheating element l2 cannot be reenergized even though the contacts 23 ofthe thermostatic switch I6 reclose when the temperature of the ironfails below the contact closing value and the iron therefore cools downto room temperature or until such a time as the operator again graspsthe handle and depresses the handle lever 43 to reclose the safetyswitch 35.

If the handle lever 43 should be released by the operator at a timeduring the thermostat heat cycle when the contacts 23 are in the open orhigh temperature position, the contacts of the safety switch will openimmediately without time delay since there will be no heating currentflowing through the bimetallic contact arm 38 to heat it and cause it tobend downwardly and maintain the contacts 36 in engagement. However, thefact that the contacts of the safety switch open immediately under thiscondition is immaterial since no current flow will be broken by thecontacts and therefore there will be no troublesome sparking.

It will be seen that my improved safety switching arrangement functionsto disconnect automatically the iron from the power supply in case it isleft unattended in the horizontal. ironing position. It will be observedthat with my improved control system the contacts of the safety switchdo not interrupt the energizing circuit of the heating element I2 exceptin a relatively -rare case when the handle lever is initially depressedand then released after a very short interval so that the bimetalliccontact arm 35 does not have a chance to heat sufficiently to maintainthe contacts of the safety switch closed. This rarely occurs, however,since, as] pointed out above, the bimetallic contact arm may be designedso that it needs to be closed not over fifteen seconds to heatsufficiently to stay closed and the times when an operator initiallygrasps and releases the handle in less than this period are rare.Therefore since the contacts of the safety switch are not required tobreak the load circuit there is no sparking so that these contacts maybe of inexpensive construction and they will operate a long time withoutdeterioration.

Furthermore, it will be noted that regardless of the frequency of theoperation of the handle lever 43 by the operator, except for the rareoccasion noted above, the energizing circuit of the heating element I2will be broken only at such times as the contacts of the thermostaticswitch move to the open circuit position incident to the normaloperation of the thermostat. I'herefore, radio interference resultingfrom sparking contacts is reduced to a minimum.

The time delay feature of providing means f or keeping the contacts ofthe safety switch closed after the handle lever is released until themain thermostat switch opens its contacts provides an additionaladvantage that when the operator temporarily releases the handle whilearranging articles to be ironed, the heating current is not immediatelycut off so that there is no tendency for the iron to cool down duringthese temporary idle periods.

While I have shown and described particular embodiments of my invention,it will occur to those skilled in the art that various changes andmodifications may be made without departing from my invention, and I,therefore, aim in the appended claims to cover all such changes andmodifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A safety control system for an electric heating device having aheating element, a circuit for energizing said heating element and anoperating handle adapted to be grasped by an operator when said deviceis being used, said system comprising a thermostatic switch responsiveto the temperature of said heating element for controlling said circuit,said thermostatic switch having a high temperature position and a lowtemperature position, a normally open safety switch connected in serieswith said circuit, a manually operated member for effecting closure ofsaid safety switch to render said thermostatic switch effctive forcontrolling said circuit, said manually operable member being arrangedto be actuated by the operator when said handle is grasped, meansdependent on the flow of current in said circuit when said thermostaticswitch is in the low temperature position for maintaining said safetyswitch closed after said manually operated member is released, said lastmentioned means becoming ineffective when said thermostat moves to thehigh temperature position whereby said safety switch opens precludingfurther control of said circuit by said thermostatic switch until saidsafety switch is again manually closed by the operator. i

2. A safety control system for an electric heating device having aheating element, a circuit for energizing said heating element, and anoperating handle adapted to be grasped by an operator l when said deviceis being used, said system comprising a thermostatic switch movablebetween open and closed positions in accordance with the temperature ofsaid element, a safety switch biased to an open position, said switchesbeing connected in series with said circuit, manually operated means forclosing said safety switch, said manually operated means being arrangedto be actuated by the operator when said handle is grasped, meansresponsive t0 the ow of current in said circuit when both of saidswitches are in the closed circuit position for maintaining said safetyswitch closed after the manually operated means is released, saidthermostatic switch moving to the open circuit position when thetemperature of said heating element rises above a predetermined valuethereby deenergizing said circuit whereby said safety switch moves tothe open circuit position precluding further energizatic-n of vsaidcircuit by said thermostatic switch until said safety switch is manuallyreclosed by the operator.

3. In combination, an electric iron having a handle, a heating elementfor said iron, an energizing circuit for said heating element, athermostatic switch responsive to the temperature of said iron forinterrupting said circuit when said temperature reaches a predeterminedhigh value and for reclosing said circuit when said temperature falls toa predetermined low value, a normally open safety switch connected inseries with said circuit, manually operated means adapted to be actuatedwhen said handle is grasped for closing said safety switch, a bimetallicmember arranged to maintain said safety switch closed when heated, andmeans for locally heating said bimetallic member in accordance with thecurrent flowing in said circuit, said means being renderedineifectiveresponsively to the operation of said thermostatic switch ininterrupting said circuit when said temperature reaches saidpredetermined high value so that said bimetallic member is permitted tocool, said member cooling at such a rate that it releases its holdingforce on said safety switch prior to the closure of said circuit by saidthermostatic switch in response to the cooling of said iron to saidpredetermined low temperature.

4. In combination, an electric iron having a handle, a heating elementfor said iron, a circuit for energizing said heating element, athermostatic switch responsive to the temperature of said iron forinterrupting said circuit when said temperature reaches a predeterminedhigh value and for reclosing said circuit when said temperature falls toa predetermined low value, a normally open safety switch connected inseries with said circuit, manually operated means adapted to be actuatedwhen said handle is grasped for closing said safety switch, a bimetallicmember operatively associated with said safety switch and movable to aposition to maintain said safety switch closed when locally heated, saidbimetallic member being arranged to be traversed and heated by thecurrent flowing in said circuit so that when said thermostatic switchattains a predetermined high temperature to interrupt said circuit saidbimetallic member cools and at such a rate that it releases its holdingforce on said safety switch prior to the reclosure of said circuit bysaid thermostatic switch responsively to the cooling of said iron tosaid predetermined low temperature.

5. In an electric heating device, a heating element, a circuit forsupplying current to said heating element and a handle for operatingsaid device, a normally open switch for controlling the ow of current insaid circuit, closing means for closing said switch when said handle isgrasped by the operator and for releasing said switch when said handleis released, means dependent upon a flow of current in said circuit forholding said switch closed independently of said closing means, meansresponsive to the temperature of said device for opening said circuitwhen said temperature rises to a predetermined high value and forreclosing said circuit when said temperature falls to a predeterminedlow value, and when said handle is released said means dependent uponcurrent iiow functioning responsively to the operation of saidtemperature responsive means in opening said circuit always to releaseits holding effect on said switch prior to the operation of saidtemperature responsive means to reclose said circuit when saidtemperature falls to said .predetermined low valut` 6. In an electricheating device, a heating element, a circuit for supplying current tosaid heating element and an operating handle that is grasped by theoperator when said device is being used, a thermostatic switchresponsive to the temperature of said device for interrupting saidcircuit when saidtemperature reaches a predetermined high value andthereafter for reclosing said circuit when said temperature falls to apredetermined low value, a normally open safety switch connected tocontrol said circuit, manually-operated means operable to a position tooperate said safety switch to close said circuit when said handle isgrasped by the operator and to a position to release said safety switchwhen said handle is released, and timing means dependent upon a Iiow ofcurrent in said circuit :for holding said safety switch to hold saidcircuit closed independently of said manually-operated means so that thesafety switch holds said circuit closed after said manually-operatedmeans releases it until said thermostatic switch opens to deenergizesaid circuit, whereupon said timing means functions to release itsholding eiect on said safety switch in a timed interval, said timingmeans being set to always release its said holding effect prior to thereclosure of said circuit by said thermostatic switch due to saidtemperature falling to said predetermined low value.

7. A safety control system for an electric fiatiron having a heatingelement, a circuit for energizing said heating element, and an operatinghandle adapted to be grasped by an operator when said iron is beingused, said system comprising a thermostat having two control positionsconnected to control the flow of current to said element, a normallyopen switch for controlling,r said circuit, manually operated means forclosing said switch, said manually operated means being arranged to 'beactuated by an operator when said handle is grasped and means responsiveto a condition iniiuenced by the position of said thermostat formaintaining said switch in a closed position independently of saidmanually operated means when said thermostat is in one of saidpositions, said last mentioned means becoming ineffective when saidthermostat moves to the other of lvsaid positions whereby said switchautomatically opens to deenergize said circuit if said manually operatedmeans has been released by the operator.

8. In combination, an electric iron having a handle, a heating elementfor said iron, an energizing circuit for said heating element, athermostatc switch responsive to the temperature of said iron forinterrupting said circuit when said temperature reaches a predeterminedhigh Value and for reclosing said circuit when said temperature falls toa predetermined low Value, a safety switch connected to control saidcircuit, manually-operated means adapted to be actuated when said handleis grasped for operating said safety switch to a position to close saidcircuit, a temperature responsive member arranged when heated tomaintain said safety switch in said position, and heating means forlocally heating said temperature responsive member in accordance withthe current flowing in said circuit, said heating means being renderedineffective responsively to the operation of said thermostatic switch ininterrupting said circuit when said temperature reaches saidpredetermined high value so that said temperature responsive member ispermitted to cool, said member cooling at such a rate that it releasesits holding force on said safety switch prior to the closure of saidcircuit by said thermostatic switch in response to the cooling of saidiron to said predetermined low temperature.

FRANK E. FINLAYSON.

